The present invention relates to transmitters used in process control loops. More specifically, the present invention relates to a temperature transmitter having a sensor with a sensor sheath lead.
Process control transmitters are used to measure process parameters in process control systems. Microprocessor-based transmitters include a sensor, an analog-to-digital converter for converting an output of the sensor into a digital form, a microprocessor for compensating the digitized output and an output circuit for transmitting the compensated output to instrumentation and control equipment. Typically, this transmission is over a process control loop, such as a 4-20 mA current loop.
One example of a process parameter is temperature. Temperature is sensed by measuring the resistance of a Resistive Temperature Device ("RTD"), such as a Platinum Resistance Thermometer ("PRT"), or by measuring a voltage output of a thermocouple sensor. For example, the resistance of an RTD can be measured by connecting the RTD in series with a known reference resistance (R.sub.REF) and applying a current common to both resistances. The voltages across each resistance are measured and digitized by the A/D converter. The microprocessor receives the digitized values, calculates the resistance of the RTD based on the digitized values and compensates the calculated resistance for various parasitic effects in the sensor and the transmitter. The resulting resistance value is converted into a corresponding sensor temperature value with a look-up table or a suitable equation by the microprocessor. The output circuit in the transmitter receives the sensor temperature value and provides the value to the process control loop as an analog current level or as a digital value superimposed on the analog circuit according to a known digital communication protocol.
A typical RTD is mounted within a distal end of a sensor sheath, which is typically a long, small diameter metal tube. A metal thermowell surrounds the sensor sheath. The RTD is electrically coupled to the transmitter's electronics through a plurality of element leads which extend from the sensor sheath. A typical RTD includes two, three or four element leads. One problem associated with RTD and other sensors is that the sensor sheath should remain electrically isolated from the sensor element and the element leads. The insulation resistance between the sensor sheath and the element leads may change over time or become shorted resulting in degraded or failed performance.
Another problem associated with temperature sensors is a parasitic sheath-to-lead voltage. A sheath-to-lead voltage is sometimes generated between the sensor sheath and the sensor element because of dissimilar metals being separated by an electrolyte made up of ceramic impurities (ionics) in the sensor and water entering into the system. This voltage may corrupt sensor measurements and result in sensor failure.
A temperature sensor and transmitter are needed which improve or substantially eliminate measurement errors from parasitic sheath-to-lead voltages and variations in insulation resistance.